The Kodansha Kanji Learner’s Course, which originally came out in Dec 2013, went out of print a couple of years ago. It seems the reason was a dispute between the author, Andrew Scott Conning Ed.D., and Jack Halpern, the owner of the CJK Dictionary Institute. This dispute went through arbitration in Japan and was decided in Conning’s favor—but Halpern didn’t pay.
The CJK Dictionary Institute is a company in Japan that develops lexical databases for Japanese and other languages, and publishes dictionaries and software derived from those databases. It also created the content for the iKnow! Japanese Core 6000 collection, which is perhaps better known as the Core 6k Anki deck. The company has received very substantial public assistance (government grants) to collect data for their proprietary databases, but seems to have shared none of its data back with the public, except by releasing paid products. I don’t know the full story about the Conning–Halpern dispute, but it sure looks like Conning got cheated. I’ve bought products from the CJK Dictionary Institute before, but I won’t do so again!
Anyhow, printed copies of the Kanji Learner’s Course were hard to come by for a while, but there are currently a small number of “new” copies for sale from third-party sellers on Amazon US, so you can order one now if you want one.